The present invention relates to a power supplying apparatus, a power line communication apparatus, a power line communication system, and a power supplying method which allow the power line communication apparatus to perform power line communication while suppressing unnecessary radiation occurring in a cable even when the power line communication apparatus, such as an imaging device or an image display device, is disposed at a place distant from an outlet.
In recent years, a demand for building a LAN in a small office or home in a simple and cheap way is increasing. In particular, a demand for building communication apparatuses, such as a network camera or a surveillance camera, capable of performing monitoring with a remote operation which is safe and reliable from the view point of crime prevention and security is rapidly increasing.
In order to build the LAN, a known method has been used in which a dedicated line for communication, such as Ethernet (registered trademark) is wired. In particular, since a wiring work is not required, a demand for a wireless LAN is increasing. On the other hand, in order to realize supply of power to the communication apparatuses and communication of the communication apparatuses by the use of only a power line, a variety of power line communication apparatuses have been proposed (for example, JP-A-2000-165304). The power line communication apparatus performs communication by superimposing a communication signal for power line communication using an OFDM (orthogonal frequency division multiplexing) method on a commercial AC voltage (for example, AC 120V, 60 Hz in U.S.A., AC 100V, 60 Hz or 50 Hz in Japan). For this reason, the power line communication apparatus requires an AC adaptor, which includes a converting circuit that supplies power to the communication apparatus and a coupler circuit that superimposes a power line signal on commercial AC power, and a power line communication modem built in the communication apparatus.
Hereinafter, a related power line communication apparatus will be described. FIG. 9 is a block diagram illustrating hardware of the related power line communication apparatus. In the case when a power line communication modem 101 receives a communication signal for power line communication, first, a commercial AC voltage is input from a power line 1 to an AC adaptor 24, which includes a coupler 68 and a signal superimposition and separation circuit 62, through an AC cable 25 such as a VVF cable. The AC adaptor 24 converts the input commercial AC voltage to a DC voltage. The communication signal for power line communication is superimposed on the DC voltage in the signal superimposition and separation circuit 62 through the coupler 68 and is then transmitted to the power line communication modem 101 and a communication terminal (not shown) through a DC cable 3.
On the other hand, in the case when the power line communication modem 101 transmits the communication signal for power line communication, the communication signal from the power line communication modem 101 is superimposed on the DC voltage and is then transmitted to the AC adaptor 24 through the DC cable 3. Then, the communication signal is separated from the DC voltage by the signal superimposition and separation circuit 62 of the AC adaptor 24; The separated communication signal is superimposed on the commercial AC voltage through the coupler 26 and is then output to the power line 1.
Since the imaging device, such as a network camera, or the image display device, such as a monitor, is disposed on a wall or ceiling in many cases, a cable having a length of 0.5 m to 10 m or more is frequently used. In particular, the imaging device is generally disposed at a variety of places for the purpose of crime prevention and security, and it is desirable to make a cable long so that the imaging device can be disposed at a place distant from an outlet.
However, in the case of applying the above-described power line communication apparatus to the imaging device or the image display device, it has been difficult to make an AC cable long so as to dispose the power line communication modem at a place distant from an outlet. Specifically, the AC cable is directly connected to a power line to which a variety of electrical apparatuses are connected. Accordingly, in the case when some of the electrical apparatuses generate noises, if the cable is long, the cable is affected due to the noises. As a result, a problem has occurred in that unnecessary radiation increases. In addition, since the commercial AC voltage is applied to the AC cable all the time, unnecessary radiation due to the commercial AC voltage occurs if a balancing control process is not performed.
On the other hand, it is possible to decrease the unnecessary radiation by making the DC cable long, as compared with a case in which the AC cable is long. However, in the case of simply lengthening the DC cable, a problem has occurred in that a signal level of a communication signal is attenuated. In this case, the signal level can be held to a certain degree by amplifying the signal level beforehand. However, the unnecessary radiation increases in correspondence with the amplification.